Optical waveguides that provide modulation of a propagating infrared laser signal are well known in the art. These devices rely on the electro-optic properties of bulk or thin film crystals to produce a change in refractive index of a traveling or standing microwave field inside the guiding crystal. The material must be a single crystal. The index of refraction of the waveguide material will vary in response to a microwave signal coupled into the waveguide, resulting in phase shift modulation of the propagating infrared beam. The phase shift modulation creates a power conversion of a portion of the infrared carrier signal into optical sideband signals.
Broadband modulation demands efficient coupling of microwave and optical fields into the waveguide. Moreover efficient interaction between these waves requires proper synchronization of the optical and microwaves such that each wavefront possesses nearly the same phase velocity. The modulated infrared beam is comprised of an input carrier frequency and the first order upper and lower sideband frequencies produced by the modulating microwave signal. Each sideband occurs within a modulation bandwidth defined as the frequency range of the optical sideband signals between the minus 3dB or half power points.
The electric field intensity required within the waveguide is such that conventional bulk electro-optic crystal modulators demand extremely high input power levels. Planar waveguide modulators fabricated from thin film electro-optic crystals have markedly improved conversion efficiencies. The same depth of modulation can be obtained with a lower modulation power as the cross sectional area of the electro-optical crystal decreases.
The planar waveguide modulators of the prior art include both standing wave and traveling wave microwave ridge modulators, as reported in an article entitled "Microwave Modulation of CO.sub.2 Lasers and GaAs optical waveguides by P. K. Cheo and M. Gilden, Applied Physics Letters, Vol. 25, No. 5, Sept. 1, 1974, pp. 272-274, and "Thin Film Waveguide Devices" by P. K. Cheo, Applied Physics, Vol. 6, pp. 1-19 (1975). These devices lack intimate electrical contact between the electrode surface of the ridge and the surface of the waveguide. Air gaps between these surfaces result from imperfect fabrication and degrade the broadband impedance match between the output impedance of the microwave source and the effective input impedance of the microwave ridge waveguide.
Additionally, the air gap causes an increase in the velocity of propagation of the microwave signal in the interaction region, since the air gap and the waveguide create a composite medium with an index of refraction less than that of the waveguide itself. This results in increased nonsynchronization of the modulating signal with the optical wave and a degradation of modulator performance.
Other devices have been developed that comprise an integrated modulator structure in which the microwave electrodes are electroplated directly on the top and bottom surfaces of a thin film optical waveguide. Although thse devices provide for greater conversion efficiency, these planar devices suffer from degradation in the optical coupling efficiency and the inability to provide more exact confinement of the optical wave through the modulation field. This results in optical distortion in the modulated wave and less than fully realizable conversion efficiency.
An improvement in these planar devices was presented in U.S. Pat. No. 4,208,091 entitled BROADBAND MICROWAVE WAVEGUIDE MODULATORS FOR INFRARED LASERS, issued to P. K. Cheo and M. Gilden. The structure of these modulators comprise a non-coplanar thin film optical waveguide wherein the guided laser propagation path includes a ridge raised from a planar surface of the device. In some embodiments, these devices additionally contain a dielectric pedestal typically comprised of GaAs that is raised from the planar surface and the ridge region. The pedestal enables wider, more easily fabricated impedance transformer electrodes to be employed.
While these structures overcome many of the shortcomings of the earlier devices and provide greater bandwidth and better frequency response, the complex three dimensional topography of these devices requires extraordinary precision and elaborate fabrication techniques. Consequently, the yield of these devices remains low and performance characteristics of individual devices have not been sufficiently uniform. In addition, maintaining reliable operation at high power levels has proved to be very difficult with these devices.